Preheat the oven to 350°. In a small bowl, mix the salt with the ancho powder and cumin. Rub the lamb shanks with the ancho mixture.

In an enameled cast-iron casserole large enough to hold the lamb shanks, heat the vegetable and olive oils until shimmering. Add the lamb shanks and cook, turning, until well-browned on all sides. Transfer the shanks to a large plate.

Reduce the heat to moderate and add the onions, garlic, celery and the carrots to the casserole. Cover and cook, stirring often, until the onions are translucent, about 10 minutes.

Return the shanks to the casserole. Add the anchos and stock and bring to a boil. Cover the shanks with a sheet of parchment paper and the lid. Braise the shanks on the bottom rack of the oven for 1 hour and 30 minutes, turning the shanks halfway through.

The shanks are done when the meat is very tender and pulling away from the bones. Transfer the shanks to an ovenproof serving dish and turn off the oven. Cover with foil and keep warm in the oven.

Set the casserole over high heat and boil the sauce until reduced to 8 cups, skimming as necessary. Working in batches, puree the braising juices and vegetables until smooth. Strain the sauce over the shanks and serve.